Modern automobile design requires light weight structures in order to comply with increasingly stringent fuel economy requirements. Regulatory requirements such as the Corporate Average Fuel Economy (C.A.F.E.) standards have put increasing pressure on automobile manufacturers to produce light weight automobiles. In addition, crashworthiness requirements are also becoming more demanding. Therefore, there is a need in the automobile industry for automobile structures having both high-strength and light-weight characteristics.
Automobile body panels are good candidates for significant weight savings in automobile structures. In typical automobile body panels, such as hoods and deck lids, a two piece design is used. An outer piece or component, typically sheet metal, serves to define the contours of the body. An inner piece or component, typically a steel stamping, serves to support the outer piece and absorb loads applied to the panel. The outer piece is joined to the inner piece at the peripheral sides of the panel. In the central portion of the panel, the outer piece is relatively loosely bonded to the inner piece with non-structural adhesive. This adhesive is applied in various regions of the inner surface of the outer piece in order to stabilize the structure of the panel. However, because the adhesive is not applied in a continuous fashion, and because the adhesive has a low modulus of elasticity, the inner and outer pieces are not tightly bonded together in the central region of the panel. As a result, under some loading conditions, the inner stamping is structurally independent from the outer contour-defining component. Thus, the inner stamping must have a higher thickness than would be required if the outer contour-defining component contributed to the stiffness of the panel.
In such panels, the inner and outer components do not effectively cooperate to absorb high loads, especially those encountered during impact. In typical automobiles, the hood is designed to "tepee," or fold upward, in a frontal impact. The purpose of such tepeeing during a frontal impact is to prevent the hood from impinging the windshield and injuring the occupants of the automobile. As such, the hood contributes a negligible percentage of the automobile's overall crash energy absorption capability. In order to make up for the lack of energy absorption of the hood, other structural components such as fenders have to be over-designed. As a result, from the standpoint of minimizing weight, the overall automobile is not as structurally efficient as may be desired.
Another problem with typical panel designs is caused by puckering of the outer piece due to shrinkage of the adhesive as it sets. The adhesive tends to flow outward as pressure is applied during the bonding process. However, the geometry of the interface between the outer and inner pieces of the panel is relatively flat, and the flow of adhesive is limited by the viscosity of the adhesive and the shear forces applied to the adhesive by the outer and inner pieces of the panel. A flat interface does not permit adhesive to flow away from the plane of the interface when pressure is applied.
Puckering is reflected by small, localized indentations which are formed on the outer surface of the panel due to the pull of adhesive on the inner surface of the sheet metal once the pressure applied during the bonding process is released and the adhesive has set. Puckering is undesirable in automobile body panels because it prevents the automobile from having a smooth, uniform finish. Puckering is aggravated when the interface between the outer and inner pieces of the panel is flat.
Yet another problem found in typical hood and deck lid panels is insufficient resistance to "oil canning." Oil canning occurs when, as pressure is applied to a body panel such as a hood, the panel "pops" in as though it were the bottom of an oil can. Oil canning is exemplified when the automobile panel is subjected to pressure loads when the owner is waxing or drying off the automobile. Oil canning at such relatively low loads is caused by a lack of adequate panel stiffness in the direction normal to the surface of the sheet metal.
Oil canning at low loads such as those applied during waxing or drying of an automobile is perceived negatively by consumers. Therefore, it is desirable to produce light weight automobile panels which have a sufficiently high oil canning stiffness so that oil canning is not exhibited during normal use of the automobile.
Palm printing is a further problem associated with automobile hood structures. Palm printing is the existence of permanent deformation in the surface of the hood due to loads applied in order to close the hood. Palm printing typically occurs when excessive downward pressure is applied to a forward end portion of the hood. Palm printing is most likely to occur if the user slams the hood when lowering the hood in order to ensure that the hood latches in the closed position. In order to ensure a smooth appearance of the hood, palm printing is to be avoided as much a possible.